Parallel Plate Electrostatic Actuation for High-Resolution ......Parallel Plate Electrostatic...

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Parallel Plate ElectrostaticActuation for High-Resolution

Deformable Mirrors

Thomas BifanoBoston University

Boston MA

8/19/04

CfAO TMT MEMS DM WS

Outline

Electrostatic MEMS DM architectures

Electrostatics and mechanics of actuators

Scaling rules and real-world limits

Case study: Extending stroke

Electrostatic MEMS DMarchitectures

Edge-supported membranes

Electrodes

Mirror

Post-supported membranes

Parallel plate (or vertical comb)actuators with supporting posts

Advantages and disadvantages

Expensive to develop

Fabrication induced stress

Limited coating options

Scalable to many actuators

High natural frequency

Batch producible

Sensitive to sound

Frame-induced warp

Low stroke @ high order

Economical to develop

Large stroke @ low order

Customizable

Controllable spatial frequencycomparison

Spatial Frequency1/D 1/(N1/2 D)

D = Mirror diameter

N = Number of actuators

Available DMamplitude

Boston University &Boston Micromachines

Corporation Design

Tip-Tilt

Piston

Continuous

To get the same AOfitting error(Kolmogorovturbulence)

Npiston/Ncontinuous = 6.2

Ntip-tilt/Ncontinuous = 1.8

(C. Max, CfAO website)

Electrostatic actuators

Simplified actuator model:

Linear Spring

Moveable plate electrode

Fixed plate electrodeV

Electrostatic attraction force

++++++++- - - - - - - -

Parallel plates form a capacitor

C =εoεrLw

εo = permittivity of free space (8.8e-12 F/m)

εr = medium dielectric constant (1 for air)

L = plate length (m)

w = plate width (m)

g = gap (m)L

Electrostatic attraction force

++++++++- - - - - - - -

Potential energy:

CV 2 =εoεrLwV 2

2gV = applied voltage (V)

Electrostatic force:

Fe =dUdg

=εoεrLwV 2

Mechanical restoring force

Fm = kx

x = go − g

x = displacement

go = initial gap

Static equilibrium: Σ F=0

Fe = Fm

εoεrLwV 2

2(go − x)2= kx

Cubic equation for x as a function of V2

0 0.5 1

Mechanical Restoring ForceE-static ForceNormalized

ForceF/Fm(max)

Normalized Displacement (x/go)

Fm = kx

FE =εoεrLwV 2

2(go − x)2

Graphical representation ofequilibrium

0 0.5 1

Mechanical Restoring ForceE-static Force @ V1

NormalizedForceF/Fm(max)

unstable

stable

0 0.5 1

Mechanical Restoring ForceE-static Force @ V1E-static Force @ V2

NormalizedForceF/Fm(max)

Increasing V

0 0.5 1

Mechanical Restoring ForceE-static Force @ V1E-static Force @ V2E-static Force @ V3Normalized

ForceF/Fm(max)

CriticalEquilibrium

0 0.5 1

Mechanical Restoring ForceE-static Force @ V1E-static Force @ V2E-static Force @ V3E-static Force @ V4Normalized

ForceF/Fm(max)

Unstable

Fe> Fm

(Newton’ssecond law:F=ma)

At critical equilibrium

Curves are tangent

εoεrLwVc

(go − xc )3= k

Fe = Fm

εoεrLwV 2

2(go − x)2= kx

εoεrLwVc

(go − xc )3xc =

εoεrLwVc

2(go − xc )2

Substitute into equilibrium eqn:

xc =go

Independent ofactuator stiffness

Critical voltage is maximumrequired to drive actuator

Vc =8kgo

27εoεrLw

Real actuator is not a parallel plate,but a bending fixed-fixed beam

k ~192EI

L3=16Etw

E = Elastic modulus

I = Moment of inertia

t = Actuator thickness

Vc ~5Et 3go

εoεrL4

Vc =5 *170e9 * (3e−6)3 * (5e−6)3

8.8e−12 *1* (3e−4 )4~ 200V

BMC µDM140:

Real actuator is not a parallel plate,but a bending fixed-fixed beam

Good news: fixed fixed beam improves stroke:

xc ~ 0.4go

The mirror adds an additionalmechanical force

Energized central actuator exerts a force on its unenergized neighbors

Influence: x2 /x1 is determined by therelative stiffness of mirror and actuator.For different BMC designs, influenceranges from 0.00 to 0.25

Voltage versus deflection isnonlinear

0 50 100 150 200

Applied Voltage (V)

Deflection (µm)

There is a voltage limitationfor parallel plate actuation

Pressure*gap (µm-atm)

Breakdown Voltage (V)

Fornitrogen

Minimum breakdown voltage as afunction of gas

Naidu, M.S. and Kamaraju, V., High Voltage Engineering, 2nd ed., McGrawHill, 1995, ISBN 0-07-462286-2

Gap at minimum(press. = 1 atm.)

Additional factors in designand performance

Film stresses (usually ~10MPa compressive) Cause actuator buckling, leading to smaller initial gap

- Buckling amplitude increases with L- Process control can help

Cause mirror nonplanarity- 30nm RMS typical, 10nm RMS achievable-1nm will require thicker mirror or different material

Spring nonlinearity (strain stiffening)As actuator deflects, the membrane lengthens

- 50MPa change in stress with 5µm deflection- This helps linearize voltage versus deflection curve!

Scaling the technology

More stroke: increase gap go or critical deflection ratio (xc/go)4.5µm is current maximum10µm is possible with current processes

More actuators: increase array size up to wafer scale (140mm)~150,000 actuatorsrequires through-wafer via connections to integrated driver electronics

Resolution

Electrostatic actuation exhibits no hysteresisOperation requires infinitesimal power (100fF capacitor)13pm repeatability measured on actuator at JPL (2nm at BU)

*Current driver has 8-bit voltage resolution, and offers resolution ofabout 1% of FS deflection.

Design modification casestudy: Double stroke

A CfAO sponsored effort

Recall that :

Vc ∝ go

32 ,L2,k

300µmL

0.4xc/go

2µmstroke

150VVc

5µmgap

Initial device design

= 1.5 ∴ Vc

= 3.7 xc

Step 1: Increase gap, by thickening actuator sacrificial layer

= 1.5 ∴ Vc

~ 2 xc

Step 2: Increase stability over a larger gap fraction, by splittingelectrodes (can get 60% of gap)

= 1.3 ∴ Vc

~ 0.6 xc

Step 3: Increase actuator length, by altering mask layout

~ 0.25 ∴ Vc

~ 0.5 xc

Step 4: Decrease actuator stiffness, by perforating membrane

~ 3.7 * 2 * 0.6 * 0.5 ~ 2.2

~ 1.5 *1.5 *1*1~ 2.2

Net result:

0 100 200 300

Voltage (V)

Deflection (nm)

Unexpected benefit:Split electrodesprevent failure dueto overvoltageinstability

Unexpected liability:RMS figure of mirrorincreased due toactuator compliance

Conclusions

Parallel plate actuation is a robust, reliable approach toMEMS DM development. Its simplicity allows deterministicdesign. It provides a proven platform for future high-resolution DMs.

Parallel Plate Electrostatic Actuation for High-Resolution ......Parallel Plate Electrostatic...

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